1. Field of the Invention
This invention relates to downhole apparatus, and in particular to apparatus including valves, such as circulating, injection or bypass apparatus.
2. Description of the Related Art
In many downhole operations, such as in the drilling of bores to access subterranean hydrocarbon-bearing formations, it is often necessary to xe2x80x9ctreatxe2x80x9d a particular section of the bore, for example to deliver fluids to stabilize a formation. In these situations it is useful to be able to deliver fluids to specific sections of a bore without having to circulate the fluids down through the length of a tubular drill string extending to the end of the bore and then from the end of the string back up through the annulus to the section of bore to be treated. Such fluid delivery is typically achieved by providing one or more injection or circulating xe2x80x9csubsxe2x80x9d in the string, which subs provide valves allowing direct communication between the string bore and the surrounding annulus.
An injection or circulatory sub will typically comprise a sleeve valve which is axially movable to uncover ports in the sub body, permitting fluid communication between the string bore and the annulus. The sleeve valve may be pressure actuated, that is the sleeve may be opened by differential pressure between the string bore and the annulus, or may include a bore restriction. The pressure differential necessary to open the sleeve is typically determined by the rating of the spring utilized to maintain the sleeve closed and the sleeve area over which the differential pressure acts.
When such a valve is xe2x80x9ccrackedxe2x80x9d, that is initially opened, the elevated pressure in the string bore will tend to cause an initial rush of fluid through the valve, and this rush of fluid may result in seals being damaged or washed out, and in erosion of the valve faces. Further, once the valve has been cracked and fluid flows from the string bore into the annulus, the pressure differential will drop, and this may result in the valve closing before opening again when the bore pressure rises once more. The valve may thus shuttle or xe2x80x9cchatterxe2x80x9d rapidly, as the differential pressure rises and falls with the closing and opening of the valve, leading to accelerated valve wear and the possibility of fatigue failure.
It is among the objectives of embodiments of the present invention to obviate or mitigate these difficulties.
According to the present invention there is provided downhole apparatus comprising: a tubular body for mounting on a tubular string; a fluid pressure actuated valve member movable relative to the body; a normally closed flow port in one of the body and valve member, with the valve member in a first position relative to the body the port being closed, and with the valve member in a second position the port being open to permit fluid communication between the body interior and exterior, the valve member being biassed towards the first position; and a latch arrangement for releasably retaining the valve member in the first position, the latch being releasable on application of a predetermined cracking pressure to the valve member, to allow the unlatched valve member to move to the second position in response to a valve opening pressure lower than said cracking pressure, and the latch being resettable on return of the valve member to the first position.
The invention is useful as a circulating or injection sub, and the latch arrangement may be selected to release at a predetermined fluid pressure force, typically a pressure differential between the string bore pressure and annulus pressure, well in excess of the pressure differential which would be encountered in normal operations, thus preventing accidental or unintentional opening; in certain well conditions, situations such as a drop in annulus pressure may result in unintentional opening of conventional apparatus. Further, in a preferred embodiment of the invention, as the latch releases, the drop in differential pressure which tends to be experienced on opening of the port will not result in the valve member reclosing the flow port.
Preferably, the latch is adapted to be resettable when the pressure applied to the valve member falls below a predetermined resetting pressure. Most preferably, the resetting pressure is selected to be lower than the valve opening pressure, such that the valve member may return to the first position, that is the valve may be reclosed, without resetting the latch. This allows the valve member to be subsequently moved to the second position, that is the valve may be reopened, in response to the valve opening pressure, rather than the higher cracking pressure. This offers the advantage that, for example, it is possible to open and close the valve on numerous occasions without having to expose a formation to an elevated cracking pressure each time. When convenient, the pressure may be reduced to the resetting pressure to reset the latch arrangement.
Preferably, the valve member is in the form of a sleeve. Most preferably, the sleeve defines a differential piston, that is there is a differential in the area of the sleeve exposed to internal fluid pressure and the area of the sleeve exposed to external fluid pressure such that a pressure differential creates an axial force on the sleeve.
Preferably also, the latch arrangement is biassed toward a latched configuration, and may be releasable as a result of movement of a latch member, which movement is resisted by a predetermined spring force. Most preferably, the latch releases the valve member following a predetermined degree of movement of the latch member to a latch release point, conveniently the latch member being axially movable to the latch release point. In a preferred embodiment the valve member defines a shoulder for engaging a latch key, on reaching the latch release point the key being radially movable out of engagement with the shoulder. Most preferably, the latch member is biassed towards the latched configuration by a spring arrangement comprising a plurality of springs arranged such that the spring compression ratings are cumulative. Conveniently, this is achieved by providing the latch member with a plurality of longitudinally spaced spring stops and the body with corresponding longitudinally spaced spring stops, with a spring located between each pair of stops. This allows a number of lighter springs to be provided to achieve a relatively high total spring rating; achieving a comparable spring rating using a single spring typically requires a spring of greater thickness thus increasing the volume which must be provided to accommodate the spring and possibly resulting in an unacceptable restriction in internal bore diameter.
Preferably also, the apparatus includes means for urging the valve member towards the first position, most preferably a spring arrangement which applies a predetermined return force to the valve member, such that a predetermined pressure differential, that is the valve opening pressure, producing a force above said return force will hold the valve open, the return force being lower than said predetermined cracking force.
Preferably also, the apparatus includes means for resetting the latch, which means may comprise a spring or other means for biasing the latch to a set position. The resetting means preferably applies a predetermined limited resetting force to the valve member such that only a relatively low predetermined pressure differential, that is a pressure differential below a resetting pressure, allows the latch arrangement to reset.
Preferably also, both the valve member and body define flow ports, which ports are alignable to allow fluid communication. Most preferably, seals are provided on one of the body and the valve member above and below the respective flow port, and the other port passes over and exposes one of the seals when the valve member moves to the second position, the seal only being exposed following unlatching or tripping of the latch; following tripping, the valve member will tend to move very quickly, under the influence of the cracking pressure force, such that the seal will only be exposed for a very short time interval.
Preferably also, the flow port seals are separated from the port by rings defining substantially circumferential slots, which slots serve to disrupt any fluid flow over the ring, and also collect any dirt or debris encountered as the valve member and body move relative to one another. The rings thus serve to protect the seals.
Preferably also, at least one of the ports defines an axially extended opening for communicating with the other port; this arrangement permits a degree of offset between the sleeve and body to accommodate, for example, a build-up of material between the body and valve member restricting movement of the valve member to the desired second position. In the preferred embodiment, the port in the body has an extended inlet opening.
Preferably also, at least one of the ports includes a flow restricting member to control the fluid flow rate through the port.
Where parts of the body and valve member slide or telescope over one another, one surface, typically a male surface, may have a relatively smooth or honed surface and the leading end of the other or female part may define a sharp edge; such an arrangement minimizes jamming or seizing of parts resulting from build-up of scale and the like on exposed surfaces.